Centrifugal mercury switch



Dec. 10, 1963 R. H. PREWITT, JR

CENTRIFUGAL MERCURY SWITCH Filed July 29, 1960 INVENTOR.

RICHARD H. PREWITT JR.

AGENT 3,114,017 CENTRIFUGAL MERCURY SWITCH Richard H. Prewitt, Ilia, Swartlunore, Pa, assignonto Burroughs tCorporation, Detroit, Mich, a corporation oi Michigan Filed July 29, 1960, Ser. No. 46,227 4 Claims. (Cl. 200-80) The invention hereinafter described and claimed has to do with electrical switches but more particularly to those of the centrifugally responsive type effective to close electrical contacts at a predetermined rotational speed.

An important object of the invention is to provide such a switch which is characterized by its reliability under all conditions of use.

Another important object is to provide such a switch which can be actuated only by a predetermined centrifugal force established by rotation about its own axis and regardless of its attitude in space.

A further object of the invention is to provide a centrifugally responsive switch with the above-mentioned characteristics and which can readily be embodied in a large variety of sizes including miniature.

More specifically it is-an object of the invention to provide a centrifugally responsive mercury switch relying upon the novel arrangement of its parts and the surface tension of the mercury to prevent closure of its switch contacts under all conditions other than when subjected to a predetermined rotational speed about its own axis.

In accordance with the above objects and first briefly described, the invention comprises a cylindrical enclosure or housing member containing a predetermined quantity of mercury, and a plurality of electrical contact elements equally spaced about and within the cylindrical walls of the housing, but exposed to the interior of the housing through restricted capillary type passageways cooperating with the surface tension of the mercury to prevent the mercury entering the passageways to interconnect the contact elements except under a predetermined centrifugal force established by rotation of the switch about its own axis. The spacing of the contact elements is so related to the quantity of mercury within the housing that in any rest position of the housing the mercury is insulficient atet More specifically and with detailed reference to the drawings illustrating the preferred embodiment of the invention, it is seen that the switch comprises a housing to bridge any two of the contact elements. This is also true when the housing is subjected to any force except that force which is equal to or more than the predetermined centrifugal force at which the switch is designed to be actuated. Thus the construction of the switch is such as to prevent closure thereof except under the desired condition.

In the drawings:

FIG. 1 is a perspective view of a switch embodying the present invention and diagrammatically showing its application in an artillery shell to condition a suitable associated circuit to effect explosion of the shell after a predetermined time interval:

FIG. 2 is a transverse sectional view through the cylindrical wall of the housing looking downwardly from over the switch, and showing its interior construction, the spacing of the contact elements around the walls thereof and the pool of mercury resting on its bottom wall;

FIG. 3 is an elevational view of the switch as it would I appear when resting on its cylindrical side wall with parts broken away to show the condition of the pool of mercury when the swtich is in this position;

FIG. 4 is a view taken along the line 4-4 of FIG. 2 and as if FIG. 2 were not in section; and

FIG. 5 is a view similar to FIG. 4 but taken along the line 55 of FIG. 2 and showing the mercury in its condition under the influence of sufficient centrifugal force to bridge and thus to electrically interconnect the contact elements of the switch.

the present invention.

10 of suitable electrically non-conductive material, including a cup-shaped base member 11 and a cap or cover member 12 tightly secured to the base as by screws 13. The base member 11 is formed with cylindrical walls 14 and a flat bottom wall 15, the latter having a recess 16 of slightly less diameter than the inner diameter of the walls 14.

"Equally spaced about the walls 14 and approximately midway betweenthe shelf 17, separating the recess 16 from the Walls 14, and the inner surface 18 of the cap member 12 are three passageways 19 of capillary dimensions, the outer ends of which are sealed by suitable plugs 20. Each of these capillary passageways is connected to a passageway 21, extending through the walls 14 from its respective interconnected passageway to the top of the wall where it is in open communication with a capillary passageway 22 extending through ribs 23 on the underside of the cover member 12 and all terminating in a common axial passageway 24 in open communication with the interior of the housing. The ribs 23 are integral with and form spoke-like members extending from an inwardly depending flange portion 25 of cover 12 having an outer diameter substantially equal to the inner diameter of walls 14 thus providing means, for correctly positioning the cover on the base member 11. The outer ends of the capillary passageways 22 are suitably sealed as indicated at 26'. Electrically conductive rods 27 forming the contact elements of the switch, extend through the cover member 12 and coaxially through the passageways 21 with their lower ends secured in wall 14. e

As seen in FIG. 4, a small quantity of mercury 2% is provided within the housing, the quantity of which is predetermined in accordance with the desired centrifugal force at which the switch is to be actuated. For example, in a miniature embodiment of the invention wherein the outer diameter of the cylindrical housing is .5", the inner I diameter is .397", the diameter of passageway 1% is 1022", and the clear inner wall dimension-exclusive of the recess 16is approximately .0937", it was found that 1.01 cc. of mercury closed the contact elements 27 at 2340 rpm. Increasing the quantity of mercury will lower the r.p.m. rate at which the contacts will be closed, while conversely raising the quantity will increase the effective r.p.m. rate. Of course another factor controlling the contact closing r.p.m'. rate is the inner diameter of the housing. While the switch is adapted for use in other apparatus, it has been herein illustrated in FIG. 1, by way of example, in association with an artillery shell 31 Such shells are fired at very high rotational speeds, such as 5,000 r.p.m.s, fully capable of actuating the switch of In the present application it is contemplated that the switch of this invention will be fixed within the nose of the shell with its axis coaxial with that of the shell whereby the rotational speed of the shell will be imparted to the switch. In this application the switch is used as an instrument in energizing a timing circuit 31 which is effective to explode the shell at a predetermined time interval after it is fired.

While the circuit does not form a part of the invention,

it will be useful in explaining its operation. Still, with tact 27c is connected to ground through a timer 36 which is also connected to ground through switch 35, when closed, and an explosive 37.

The rotational speed imparted to the switch by the spinning projectile will cause the mercury to climb up the inner walls of the housing member 11 and spread evenly around these walls, as seen in FIG. 5, but until the housing reaches the rotational speed at which it is desired to interconnect the contacts by means of the mercury, the capillary restriction of the passageways 19 and the surface tension of the mercury will prevent the mercury from entering these passageways to make contact with the con-- tact elements. However, when the desired rotational speed is reached, which, of course, in such a projectile: is almost immediately, the surface tension of the mercury will be broken and the mercury will flow into the passageways 19 and electrically interconnect the contacts 27. Air within the passageways 1% is forced through passageways 21 and 22 to re-enter the interior of the housing through the common passageway 24. Closure of contact 27a provides the power supply from battery 32. Closure of contact 2712 explodes the charge 34 to close switch 35 which readies the timing mechanism circuit. Finally closure of contact 270 starts the timing mechanism to effect explosion of the charge 37 after a timed interval determined by the setting of the timer 36.

It can readily be understood that it is important the switch not be closed under any other conditions than when fired. The switch construction of the present invention prevents this from happening.

With reference to FIG. 3 illustrating the switch structure as if resting upon its cylindrical wall 14, it is seen that the pool of mercury 23 is insuiiicient to bridge two of the adjacent contacts 27 and thus it is understood that if the switch or the apparatus to which it is attached, such as the shell 39, is dropped that no two of the contacts will be bridged by the mercury. Of course, it is realized that the switch may be dropped when a single contact 27 is in a position under the mercury, but contact of the mercury with only one of the contacts would be ineffective.

As seen in FIG. 4, it is obvious that the pool of mercury is insufficient toeffect closure of the contacts should the projectile be dropped with the bottom wall 15 of the switch facing downwardly. While not illustrated, it is also obvious that dropping of the projectile with the cover member 12 of the switch facing downwardly that because of the restricted diameter and the length of these passageways the mercury would be insuflicient to flow through the capillary passageways 22 and 2.4 to reach the electrodes 27.

From the above description it is seen that the invention provides a fool proof centrifugally responsive switch which is operable at any attitude in space when subjected to a predetermined centrifugal force about its own axis, but which cannot operate at lesser rotational speeds, or by impact or gravity.

What is claimed is:

1. A centrifugally responsive switch comprising a housing of electrically non-conductive material and including a cylindrical side wall having an axis and walls closing the ends of said cylindrical wall, a plurality of electrically conductive contact elements equally spaced around and within said side wall with portions thereof exposed on the exterior of said Wall for interconnection with associated apparatus and other portions in open communication with the interior of said housing through capillary passageways in said cylindrical wall substantially midway between said end walls and extending through said cylindrical wall and one of said end walls to terminate in open communication with the interior of said housing at a point coaxial with said axis, and a pool of mercury within said housing, said mercury being of a quantity sufiicient only in response to a predetermined centrifugal force generated .by rotation of the housing about its axis to spread itself around the cylindrical wall and into the capillary passageways thus to bridge and interconnect said contact elements, the extension of said capillary passageways through said cylindrical wall and said end wall serving to provide means for permitting the air within the capillary passageways to flow in advance of the mercury and to exhaust into the housing through the axial termination in said end wall.

2. A centrifugally responsive switch comprising a housing of electrically non-conductive material and including a cylindrical side wall having an axis and walls closing the ends of said cylindrical wall, three electrically conductive contact elements equally spaced around and within said side wall with portions thereof exposed on the exterior of said wall through one end wall thereof for interconnection with associated apparatus and other portions in open communication with the interior of said housing through radially extending capillary passageways in said cylindrical wall substantially midway between said end walls and extending axially through said cylindrical wall and radially through one of said end walls to terminate in open communication with the interior of said housing at a point coaxial with said axis, and a pool of mercury within said housing, said mercury being of a quantity sufiicient only in response to a predetermined centrifugal force generated by rotation of the housing about its axis to spread itself around the cylindrical wall and into the radially extending capillary passageways in said cylindrical wall thus to bridge and interconnect said contact elements, the extension of said capillary passageways through said cylindrical wall and said end wall serving to provide means for permitting the air within the capillary passageways to flow in advance of the mercury and to return to the housing through the axial termination in said end wall.

3. A centrifugally responsive switch comprising a housing of electrically non-conductive material and including a cylindrical side wall having an axis and walls closing the ends of said cylindrical wall, three electrically conductive contact rods equally spaced from each other around and axially positioned within said side wall with portions thereof extending exteriorly through one end wall for interconnection with associated apparatus and other portions in open communication with the interior of said housing through radially extending capillary passageways in said cylindrical wall substantially midway between said end walls and extending through said cylindrical wall coaxially with said contact rods and radially through ribs formed in one of said end walls to terminate in open communication with me interior of said housing at a point on said last mentioned end wall coaxial with said axis, and a pool of mercury within said housing, said mercury being of a quantity sufficient only in response to a predetermined centrifugal force generated by rotation of the housing about its axis to spread itself around the cylindrical wall and into the radial capillary passageways in said side wall thus to bridge and interconnect said contact elements, the axial extension of said capillary passageways through said cylindrical wall and the radial extension through the ribs of said end wall serving to provide means for exhausting air within the capillary passageways displaced by said mercury to how in advance of the mercury and to return to the housing through the axial termination in said end wall.

4. A centrifugally responsive switch comprising a housing of electrically non-conductive material bonded by a cylindric wall and parallel end walls, said housing formed with a plurality of capillary passageways each opening at one of its ends from the interior surface of said cylindric wall and extending integrally into an end wall to terminate at the interior surface of the latter, electrically conductive contact elements, each having a first portion thereof received in a cylindric wall portion of a passageway and in non-blocking relationship therewith, and a second portion thereof for interconnection with apparatus exterior of said housing, and a quantity of electrically conductive fluid within said housing sufficient only in response to a predetermined centrifugal force generated by rotation of the housing about the axis of its cylindric wall to spread itself around the interior surface of the latter and into the capillary passageways thus to bridge and electrically interconnect said contact elements, said end Wall portions of said capillary passageways serving 5 to provide means for permitting air Within said passage- Ways to flow in advance ofsaid electrically conductive fluid and to exhaust into the housing. a

References Cited in the file of this patent UNITED STATES PATENTS Von Bornhard Dec. 22, 1959 

1. A CENTRIFUGALLY RESPONSIVE SWITCH COMPRISING A HOUSING OF ELECTRICALLY NON-CONDUCTIVE MATERIAL AND INCLUDING A CYLINDRICAL SIDE WALL HAVING AN AXIS AND WALLS CLOSING THE ENDS OF SAID CYLINDRICAL WALL, A PLURALITY OF ELECTRICALLY CONDUCTIVE CONTACT ELEMENTS EQUALLY SPACED AROUND AND WITHIN SAID SIDE WALL WITH PORTIONS THEREOF EXPOSED ON THE EXTERIOR OF SAID WALL FOR INTERCONNECTION WITH ASSOCIATED APPARATUS AND OTHER PORTIONS IN OPEN COMMUNICATION WITH THE INTERIOR OF SAID HOUSING THROUGH CAPILLARY PASSAGEWAYS IN SAID CYLINDRICAL WALL SUBSTANTIALLY MIDWAY BETWEEN SAID END WALLS AND EXTENDING THROUGH SAID CYLINDRICAL WALL AND ONE OF SAID END WALLS TO TERMINATE IN OPEN COMMUNICATION WITH THE INTERIOR OF SAID HOUSING AT A POINT COAXIAL WITH SAID AXIS, AND A POOL OF MERCURY WITHIN SAID HOUSING, SAID MERCURY BEING OF A QUANTITY SUFFICIENT ONLY IN RESPONSE TO A PREDETERMINED CENTRIFUGAL FORCE GENERATED BY ROTATION OF THE HOUSING ABOUT ITS AXIS TO SPREAD ITSELF AROUND THE CYLINDRICAL WALL AND INTO THE CAPILLARY PASSAGEWAYS THUS TO BRIDGE AND INTERCONNECT SAID CONTACT ELEMENTS, THE EXTENSION OF SAID CAPILLARY PASSAGEWAYS THROUGH SAID CYLINDRICAL WALL AND SAID END WALL SERVING TO PROVIDE MEANS FOR PERMITTING THE AIR WITHIN THE CAPILLARY PASSAGEWAYS TO FLOW IN ADVANCE OF THE MERCURY AND TO EXHAUST INTO THE HOUSING THROUGH THE AXIAL TERMINATION IN SAID END WALL. 